1. Field of the Invention
The invention relates to enantiomerically enriched malonic acid monoesters substituted by a tertiary hydrocarbon radical, and their salts, and to a process for their preparation from the corresponding prochiral malonic acid diesters by enzymatic partial hydrolysis (or partial saponification).
2. Description of the Background
Enantiomerically enriched compounds are of considerable importance as synthetic units in the pharmaceutical and agrochemical sectors, and as chiral auxiliaries. Possible starting materials for enantiomerically enriched xcex1-monosubstituted or xcex1,xcex1-disubstituted malonic acid monoesters are the corresponding diesters, which contain a prochiral carbon atom with two identical carboxylic acid ester groups and two other substituents which differ from one another and from the carboxylic acid ester groups. Partial saponification to the monoester produces a center of chirality and enantioselective partial hydrolysis gives enantiomerically enriched and, in the most favorable case, practically enantiomerically pure malonic acid monoesters.
xcex1,xcex1-Disubstituted malonic acid diesters can be selectively converted to enantiomerically enriched malonic acid monoesters with the aid of porcine liver esterase (M. Schneider et al., Angew. 96 [1984], 54). This gives good enantiomeric excesses when there is a marked difference in size between the substituents. xcex1-Chymotrypsin is another useful enzyme for the enantioselective partial saponification of xcex1,xcex1-dialkylnialonic acid diesters to the corresponding xcex1,xcex1-dialkylmalonic acid monoesters (F. Bjxc3x6rkling et al., Tetrahedron, 41 [1985], 1347).
This known enzyme-catalyzed partial hydrolysis could not be applied to xcex1-monoalkylmalonic acid diesters. T. Kitazume et al. (J. Org. Chem., 51 [1986], 1003) were not able to obtain significant enantiomeric excesses because the monoesters in question immediately racemized under the reaction or working-up conditions.
A. L. Gutman et al. (J. Org. Chem., 57 [1992], 1063) obtained benzyl methyl xcex1-methoxymalonate from dimethyl xcex1-methoxymalonate with an enantiomeric excess of 98% by transesterification with benzyl alcohol using the lipase from Candida cylindracea as the transesterification catalyst. Monomethyl xcex1-methoxymalonate could then be obtained by catalytic hydrogenation. This procedure is restricted to the methoxy compound and, as a two-stage process, is relatively expensive. Moreover, the configuration of the monoester is only stable in organic solvents.
The invention now provides enantiomerically enriched malonic acid monoesters xcex1-monosubstituted by a tertiary hydrocarbon radical, or their salts, of the general formula I: 
in which R1, R3, R4 and R5 are identical or different hydrocarbon radicals, and wherein any two of the radicals R3, R4 and R5 may alternatively be present as a carbocyclic ring together with the quaternary carbon atom which they substitute, and M is hydrogen, one equivalent of a metal or an optionally substituted ammonium ion.
The invention further provides a process for the preparation of the enantiomerically enriched malonic acid monoesters xcex1-monosubstituted by a tertiary hydrocarbon radical, or their salts, of the above general formula I, which comprises an enzymatic partial hydrolysis of xcex1-monosubstituted malonic acid diesters of the general formula II: 
in which R1, R3, R4 and R5 are defined as indicated for the formula I and R2 is also a hydrocarbon radical, which can differ from R1.
The process according to the invention produces the xcex1-monosubstituted malonic acid monoesters I and their salts with good yields and a high enantioselectivity starting from the corresponding xcex1-monosubstituted malonic acid diesters II, which are inexpensive and also readily available in industrial quantities. The tertiary hydrocarbon radical of the xcex1-monosubstituted malonic acid diester II, with its quaternary carbon atom bonded to the a carbon atom of the malonic acid diester II, obviously allows enantioselective hydrolysis of only one of the carboxylic acid ester groups. The configuration of the enantiomerically enriched xcex1-monosubstituted malonic acid monoesters is stable over a wide pH range. This is surprising because, according to the literature (T. Kitazume et al., loc. cit.), monosubstituted malonic acid diesters give racemic products on enzymatic saponification.
In the preferred xcex1-monosubstituted malonic acid monoesters I, and accordingly also in the xcex1-monosubstituted malonic acid diesters II preferred as starting materials, R1 is a lower alkyl radical having 1 to 4 carbon atoms, especially 1 or 2 carbon atoms, or the benzyl radical and R3, R4 and R5 are identical or different and are alkyl, alkenyl, aryl, alkaryl or aralkyl radicals having up to 10 carbon atoms. If any two of the radicals R3, R4 and R5 form a carbocyclic ring with the quaternary carbon atom which they substitute, this ring preferably has a saturated hydrocarbon structure with 5 to 12 ring members, especially 5 or 6 ring members. M in the formula I is preferably hydrogen, one equivalent of an alkali metal or alkaline earth metal or an optionally alkyl-substituted ammonium ion. R2 in the formula II has the same preferred meaning as R1 and can differ from R1.
If all three substituents R3, R4 and R5 are different, the malonic acid esters have a center of chirality, so the xcex1-monosubstituted malonic acid diesters II can be obtained as racemates or pure enantiomers. Both are suitable as starting materials for the process according to the invention. In the case of racemates, the enzymatic partial hydrolysis proceeds as a kinetic resolution of the racemates, thereby influencing the magnitude of the enantiomeric excess which can be achieved by the process of the invention. The enantiomeric excess is generally from 80 to  greater than 99%, depending on the substituents R3, R4 and R5. The xcex1-monosubstituted malonic acid diesters can be prepared from the unsubstituted malonic acid diesters in known manner by reaction with the appropriate electrophiles.
Examples of suitable malonic acid diesters II which may be mentioned are diethyl xcex1-tert-butylmalonate, dimethyl xcex1-tert-butylmalonate, dibenzyl xcex1-tert-pentylmalonate, dimethyl xcex1-tert-pentylmalonate, dimethyl xcex1-(1-methyl-1-phenylethyl)malonate, diethyl xcex1-(1-methyl-1-phenyl-n-propyl)malonate, dibutyl xcex1-(1 -ethyl-1 -phenylethyl)malonate, diethyl xcex1-(1-methylcyclohexyl)malonate and diethyl xcex1-(1,1-dimethylprop-2-enyl)malonate.
Any known enzymes which hydrolyze carboxylic acid ester groups in aqueous media can be used as catalysts for the enzymatic partial hydrolysis. Any commercially available ester-cleaving enzymes, including esterases, lipases and proteases, are suitable for this purpose. They can be used in crystalline form, in aqueous suspension or fixed to a support. Examples of suitable enzymes include porcine liver esterase, the lipase from Candida cylindracea and xcex1-chymotrypsin, already referred to above, the papain from Carica papaya and the lipase from porcine pancreas.
The process according to the invention is conveniently carried out by suspending the xcex1-monosubstituted malonic acid diester II in water, an aqueous solution or a buffer solution affording optimal pH adjustment. Examples of suitable buffers are phosphate buffer, citrate buffer and tris(hydroxymethyl)aminomethane (abbreviated to xe2x80x9cTrisxe2x80x9d). They are generally used in concentrations of 0.01 to 3 M and conveniently in a weight ratio of 2 to 50, relative to the xcex1-monosubstituted malonic acid diester II. The addition of a solvent, for example in amounts of up to about 40 percent by weight, based on the aqueous buffer solution, is helpful in many cases. Examples of suitable solvents are ethanol, tetrahydrofuran, dimethyl sulfoxide, N-methylpyrrolidone and acetonitrile.
The partial hydrolysis according to the invention takes place under the mild conditions typical of enzymatic reactions. The pH is advantageously kept in the range from about 4 to about 9 by the addition of a solution or suspension of a basic substance which provides hydroxyl ions. The optimal pH depends essentially on the enzyme used and can easily be determined by means of preliminary experiments. Examples of suitable basic substances are metal hydroxides or optionally substituted ammonium hydroxide. Dilute solutions, i.e., 0.05 to 2 N solutions, of alkali metal or alkaline earth metal hydroxides, alkali metal carbonates and/or alkali metal hydrogen carbonates are preferred. Appropriately diluted aqueous solutions of ammonia and of primary, secondary or tertiary alkylamines, such as methylamine, diethylamine or tributylamine, are also preferred as solutions which provide hydroxyl ions. The hydroxyl ions are required in molar quantities so that the xcex1-monosubstituted malonic acid monoesters I are obtained in solution in the form of their corresponding salts at the end of the reaction.
The partial hydrolysis according to the invention generally takes place at atmospheric pressure, although it is also possible to apply elevated or reduced pressures, e.g. 0.7 to 2 bar. The hydrolysis remains partial under the conditions indicated, i.e., the second carboxylic acid ester group is not attacked.
The process can be carried out batchwise or continuously. In the batch procedure, it is possible, e.g., to place the xcex1-monosubstituted malonic acid diester, the buffer solution, the enzyme and optionally a solvent in a stirred vessel, bring the mixture to the desired temperature and ensure thorough mixing by stirring. The amounts used in this procedure range from nanomoles to about 0.5 mol of xcex1-monosubstituted malonic acid diester II per mg of enzyme. The pH of the mixture is monitored and kept in the indicated range by the addition of a solution which provides hydroxyl groups. The reaction mixture can be worked up by acidifying, e.g., with sulfuric or hydrochloric acid, filtering off the solid constituents, including the enzyme, and recovering the xcex1-monosubstituted malonic acid monoester in solid form by extraction of the acid solution with a water-insoluble solvent, such as diethyl ether, and evaporation of the solvent from the ether solution, which has conveniently been dried beforehand. An alternative possibility is to separate off the enzyme by centrifugation or by means of a membrane process and then acidify the solution.
The enantiomeric excess can be determined in known manner, e.g., by converting the xcex1-monosubstituted malonic acid monoester to the acid chloride, for example by means of cyanuric chloride, reacting said acid chloride with an optically active amine, for example, with S-phenylethylamine, to give the carboxamide, determining the ratio of the isomers by quantitative gas chromatographic analysis and using this to calculate the enantiomeric excess. The enantiomerically pure xcex1-monosubstituted malonic acid monoesters can be obtained from the enantiomerically enriched products by means of conventional enantioselective or diastereoselective crystallization processes (E. L. Eliel, S. H. Wilen and L. N. Manden, Stereochemistry of Organic Compounds, John Wiley, 1994).
When the product separated from the reaction mixture as described, or an enantiomerically pure product, is dissolved in an aqueous solution of a basic substance, as illustrated previously, the corresponding salt of the xcex1-monosubstituted malonic acid monoester I is obtained; said salt can be isolated as the solid product by evaporation of the solution, conveniently under vacuum, or reacted further in the solution.
The process according to the invention can be carried out continuously, e.g., by working in an enzyme membrane reactor by known methods or allowing a mixture of the buffer solution, the xcex1-monosubstituted malonic acid diester II and optionally a solvent to trickle over an immobilized enzyme fixed in a reaction tube, with continuous monitoring of the pH, and working up the product stream as described previously, whereby separation of the enzyme naturally becomes superfluous.